Laboratory Investigation of Enhanced Light-Oil Recovery By CO/Flue Gas Huff-n-Puff Process

Abstract

This paper focuses on phase behaviour measurements with reservoir oil-CO ₂ mixtures and on coreflooding tests in the huffn- puff mode to characterize the system, determine the influential mechanisms, and supply data for simulation of the field implementation. The results indicate that significant amounts of CO ₂ could dissolve in the oil, which caused oil swelling and viscosity reduction. During the puff cycle, the oil retained CO ₂ preferentially to methane; thus, the beneficial swelling and viscosity effects were maintained over an extended portion of this cycle. Corefloods were performed to investigate the effect of waterflood residual oil saturation and injection gas composition (CO ₂ and enriched flue gas) on oil recovery. Incremental oil recovery was observed to be sensitive to waterflood residual oil saturation and to the process application scheme. Coreflooding results suggest that the huff-n-puff process may be more suitable to oil-wet than water-wet reservoirs.

Introduction

Various technologies have been applied in tertiary oil recovery processes, such as gas miscible/immiscible injection and chemical flooding. Among these enhanced oil recovery (EOR) methods, the huff-n-puff process has been reported to be economic at an oil price of less than US$20/STB and CO ₂ costs of US$40/ton ⁽¹⁾. For example, it was shown in a flue-gas huff-n-puff project ⁽²⁾that oil production rates stabilized, and the project proved to be cost effective with small investment requirements and low operating costs. In another case ⁽³⁾, the CO ₂ huff-n-puff process was not successful in increasing incremental oil recovery. However, there were reduced water-handling and electrical requirements during the injection, soak, and flow phases, which were beneficial to the project.

There is increasing interest in CO ₂/flue-gas huff-n-puff injection into single wells because the process is relatively easy to apply and does not require a large initial capital outlay. The process typically begins with the injection of a slug of gas into a single well. This is followed by a shut-in or soak period to allow the gas to dissolve into the oil, swell its volume, and reduce its viscosity. The same well is then returned to production and the response is monitored. In reservoirs with poor inter-well communication, this single-well approach may be one of the best ways, and sometimes the only way, to accelerate response in underperforming wells. Since miscibility between the reservoir oil and injected gas is not a requirement of the huff-n-puff process, it is well suited for low pressure reservoirs and for gases with high minimum miscibility pressures such as flue gas.

The mechanisms involved in the production of oil during gas huff-n-puff are diverse and complex. The following mechanisms have been mentioned in the literature ^(4-6): a) oil viscosity reduction; b) oil swelling; c) solution gas drive; d) relative permeability hysteresis due to reduced water saturation, drainage/imbibition, and wettability alternation; e) repressurization; f) gas diffusion and mass transfer; and, g) interfacial tension reduction in the zone near the wellbore.

The purpose of this work is to investigate the potential for applying the CO ₂ huff-n-puff process in a medium-gravity oil reservoir in Saskatchewan.